According to the explosive proliferation of the Internet, increases in the transmission capacity of optical communication systems are proceeding rapidly with the United States playing a central role. The key technique for increasing the transmission capacity is the wavelength division multiplexing (WDM) system. The WDM system has the capability of transmitting plurality of optical signals with different wavelengths over a single optical fiber, thereby increasing the communication capacity markedly. Although optical communication systems now installed are point to point systems that interconnect nodes individually, optical cross connect systems and optical add/drop multiplexing (OADM) systems with higher functions are now being developed. The optical cross connect system is a system that carries out switching of transmission lines at a node such as a telephone office. On the other hand, the optical add/drop multiplexing system is a system that extracts a specified wavelength from multiplexed signals to distribute it to another transmission line, or adds a new signal to the specified wavelength to be sent. Besides the ability to multi/demultiplex the optical signals, these systems must have spatial division optical switches that can switch the optical paths without converting the optical signals into electrical signals. Thus, the space division optical switch is one of the important components of a future optical communication network.
The optical switches used for these commercial communication systems must have such characteristics as small size, low cost, low power consumption and fast controllability. In addition, optical switches are required which have transmittance with small wavelength dependence and polarization dependence, and is switchable over a broad wavelength band.
While research and development of a variety of optical components have been conducted, waveguide-type optical components based on optical waveguides formed on a substrate is receiving attention as optical components superior in mass-producibility, integratability and reliability. The waveguide-type optical switches are mass-produced at high accuracy and at high reproducibility less than or equal to the order of wavelength by photolithography and microprocessing, and are suitable for very large systems because basic elements constituting the switches are small in size. Thus, they are considered to be most promising optical switches. In particular, silica-based optical waveguides formed on a silicon substrate are low in loss, and have high reliability and extensibility. Thus, they are expected as a component of future optical communication systems.
As a basic constituent of the conventional waveguide-type optical switches, a Mach-Zehnder interferometer is used. The Mach-Zehnder interferometer has two couplers and arm waveguides connecting the two couplers. Driving a thin film heater on the arm waveguides enables switching.